The present invention relates to printed wiring boards and flexible circuitry and, more particularly, relates to methods of manufacture of novel and inexpensive printed wiring boards and flexible circuitry. The invention discloses a direct mode and a transfer mode of making the inexpensive printed wiring boards.
World wide the printed wiring board (PWB) and flexible circuit industry is about a $25 billion business. Methods for manufacturing printed wiring boards and flexible circuits have remained largely unchanged since the industry""s origin in the 1950s. The fabrication process, generally known as photo-lithography, is largely a subtractive process: broad area copper foil is first protected by a pattern etch-resistant resin; and then the remaining unprotected area on the copper foil is etched away, usually by a liquid/chemical etching process. This method involves numerous time-consuming and costly intermediate steps. Before the copper is etched, the resist must be laminated, exposed, and developed before the copper can be etched. In addition, the resist on the copper surface must be stripped after the etching process is completed. Finally, the entire board must be washed, dried, cured, and baked at various times during the fabrication process. Despite the tedious and time-consuming nature of the process, photo-lithography is the dominant fabrication process in both the printed wiring board and flexible circuit industry and in the semiconductor industry.
Electrostatic printing, sometimes referred to as Xeroprinting, typically utilizes an electrostatic printing plate or roll including a grounded conductive substrate with a permanent (persistent or fixed) image or pattern of insulating material formed thereon. A common method of forming the permanent image or pattern on the surface of the conductive substrate is to deposit a photosensitive polymer layer, also referred to as a photo-polymer layer, on the surface of the conductive substrate, such as disclosed in U.S. Pat. No. 4,732,831 to Riesenfeld, et al. Such layer is typically 5 to 50 xcexc thick. The photo-polymer is exposed to actinic radiation in a desired image or pattern causing the photo-polymer to selectively increase its resistivity, producing a persistent image on the electrostatic printing plate. Thereafter, the electrostatic printing plate is charged using corona discharge, causing the latent, high resistivity, insulating areas to build a static charge, while areas of low resistance discharge comparatively quickly. The image is developed for transfer to another surface by toning with oppositely charged particles of toner, in liquid or dry form. The toner is then transferred by electrostatic or other means to another surface such as paper, polymeric film or phenolic resin. Since the original image is fixed in the photo-polymer layer, multiple copies can be made with a single exposure of the photo-polymer by merely repeating the corona charging, toning and transfer steps.
A mask or photo-tool may be utilized to expose the electrostatic printing plate to light. When the photo-polymer layer of the printing plate is exposed to actinic radiation through the mask or photo-tool, the polymeric molecules of the photo-polymer become cross-linked in the pattern exposed and an image or pattern is developed in the photo-polymer. When the electrostatic printing plate is charged with a corona unit, of the type known in the art, the cross-linked regions of photo-polymer retain a high level of electrostatic charge, but the unexposed, uncross-linked regions quickly dissipate the charge. Alternatively, a photo-polymer may be selected that reduces cross-linking when exposed to actinic radiation, which likewise produces a persistent image or pattern of contrasting high-resistivity and low-resistivity regions on the surface of the electrostatic printing plate.
Over the years, there have been numerous attempts to improve on the subtractive photolithographic processes. These attempts have focused on either improving the liquid etching process, or sidestepping the etching process entirely by developing an xe2x80x9cadditivexe2x80x9d fabrication process. However, the improved etching baths build up with metal sludge, and need to be periodically disposed of and replaced at considerable cost. The attempts at developing an additive process use many xe2x80x9celectrolessxe2x80x9d plating steps and involve toxic and complex chemistries. The electroless plating steps are generally slow and produce copper of poor quality, resulting in a lower quality board and are incapable of producing thick copper layers.
Accordingly, prior to the development of the present invention, there has been no method for manufacturing high-quality printed wiring boards and flexible circuits in a quick and efficient manner at low cost.
The invention describes inexpensive methods for manufacturing printed wiring boards and flexible circuits. Specifically, an electrostatic printing plate is employed and used as an imaginable dielectric to which an electrically conductive toner, such as, a silver toner, is applied. The toner is then cured. Finally, bulk conductor metal, preferably copper, is electroplated on the silver toner conductor patterns to finish the process. Alternatively, the silver toner is transferred directly to a dielectric surface and processed to form the PWB or the flexible circuit.
It is an object of the present invention to selectively form a toner layer pattern on an electrostatic printing plate on which a copper layer is formed.
The electrostatic printing plate can be rigid or flexible and is coated with an image receiving layer. The image receiving layer is preferably comprised of a photopolymer material.
The invention also relates to an electrostatic printing plate or drum that is fabricated using a process that causes an amorphous, insulating layer to selectively crystallize in a desired pattern, which can then be used to repeatedly transfer dry or liquid toner to a receiving surface.
In one preferred embodiment, an electrostatic printing plate includes a metal substrate with at least one photopolymer layer deposited thereon. A silver containing toner is subsequently deposited on the photopolymer layer in a desired image. The photopolymer material layer can then be heated to an adequate temperature for the desired time causing a pattern to develop in the photopolymer layer. The resulting layer is highly durable and allows millions of images to be transferred from a single electrostatic plate or drum.
Alternatively, a mask could be used to selectively deposit quantities of toner, such as, a silver toner, on the surface of either the substrate or the photopolymer material layer.
The present invention is further directed to a method of fabricating the electrostatic printing plate.
Further objects and advantages of the present invention will be apparent to those skilled in the art from the detailed description of the disclosed invention.